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Small Backfire and Low Compression on Middle Cylinders


Acgallen

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I recently did a full head gasket replacement to fix it blowing white smoke out of the exhaust, which worked. Now after putting everything back together ive noticed a few small problems. I bought the truck knowing it ran but i never layed on the gas because of the bad head gasket, so not sure if the backfire was from the fix or it was from before. I only notice the backfire when i really give it gas, and its a low popping noise with the occasional loud one. I did a compression test on all the cylinders and found 4 were at 160-165 and the middle 2 were at 140-145. The middle drivers side spark plug was completely black and not sure what that's from either, as all the others are brand new clean plugs. Any help is appreciated, i can try to give more info but i only know so much.
 


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rusty ol ranger

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140-160psi spread isnt bad. 20% is generally the rule of thumb.

But the backfireing is concerning...did you take a good look at the valves when you had the head off? Did you torque everything correctly?
 

Acgallen

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They looked fine when i had it out, made sure everything went back the same. Torqued everything to manufacturing specs and the right pattern, thats why im scratching my head on this one.
 

rusty ol ranger

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Is it backfiring out the intake or exhaust?

If the exhaust is popping you may have a rich condition. If youre getting to much fuel the popping noise is raw fuel exploding in the hot manifold.

If its coming out the intake you might jumped time.
 

Acgallen

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Not quite sure, how would be the best way to test that?
 

rusty ol ranger

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If that one plug is black that means that cylinder is rich as hell. Either a stuck injector dumping fuel or its not getting good (or any) spark.
 

Acgallen

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Ill definitely check that out and see if that fixes the issue
 

Acgallen

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Cleaned out all fuel injectors, didnt fix the backfire but it runs a little better
 


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